Hey there!
You can think of these questions in terms of dollar bills. For example, "tens" can also be ten dollar bills. So, three tens would be:
$10 + $10 + $10 = $30
Now, multiply that number of dollars by 10. Remember a really good trick for doing mental math with multiplying by 10: just add a zero onto the end of the other number.
10 × $30 = 300
Your answer will be 300.
Hope this helped you out! :-)